A honeycomb filter has been used in a capturing filter of particulates, particularly diesel particulates in an exhaust gas from an internal combustion engine, a boiler or the like.
In general, as shown in FIGS. 8(a), 8(b), the honeycomb filter for use in this purpose has a structure which includes a large number of through channels 3 partitioned by partition walls 2 and extending through an X-axis direction and in which the adjacent through channels 3 are plugged in one end alternately on opposite sides so that end surfaces have a checkered pattern. In the honeycomb filter having this structure, a fluid to be treated flows in the through channel 3 whose inlet end surface 42 is not plugged, that is, the through channel 3 whose outlet end surfaces 44 is plugged, passes through the porous partition wall 2, and is discharged via the adjacent through channel 3 whose inlet end surface 42 is plugged and whose outlet end surface 44 is not plugged. In this case, the partition walls 2 form the filter, and soot discharged, for example, from a diesel engine or the like is captured by and deposited on the partition walls. The honeycomb filter used in this manner has problems that a temperature distribution in the honeycomb structure becomes nonuniform by a rapid temperature change of an exhaust gas or a locally generated heat and that the honeycomb filter is cracked. Especially, with the use as the filter (hereinafter referred to as DPF) which traps a particulate material in the exhaust gas of the diesel engine, it is necessary to regenerate the filter by combusting and removing accumulated carbon particulates. In this case, there is a problem that temperature is locally raised, regeneration efficiency drops by unevenness of regeneration temperature, and cracks easily occur by a high heat stress. Since the temperature distribution during a regeneration is not uniform, it is difficult to set the whole filter at an optimum temperature and to enhance the regeneration efficiency.
To solve the problem, a method has been proposed in which a plurality of segments obtained by dividing the honeycomb filter are bonded by bonding materials. For example, in U.S. Pat. No. 4,335,783, a method of manufacturing a honeycomb structure has been described in which a large number of honeycomb members are bonded via discontinuous bonding materials. Moreover, Japanese Patent Publication No. 61-51240, a thermal shock resistant rotary heat accumulation system has been proposed in which matrix segments of the honeycomb structure formed of a ceramic material are extruded/formed and fired. Subsequently, an outer peripheral portion of the structure is processed and smoothed. Therefore, a bonded portion is coated with a ceramic bonding material whose mineral composition after the firing is substantially the same as that of the matrix segment and whose difference in coefficient of thermal expansion is 0.1% or less at 800° C., and fired. Moreover, in SAE paper 860008, 1986, a ceramic honeycomb structure has been described in which the honeycomb segments of cordierite are similarly bonded with cordierite cement. Further in Japanese Patent Application Laid-Open No. 8-28248, a ceramic honeycomb structure has been described in which a honeycomb ceramic member is bonded with an elastic seal material formed of inorganic fibers crossing one another in at least three dimensions, an inorganic binder, an organic binder, and inorganic particles. Moreover, a prevention against a honeycomb filter failure caused by a thermal stress has been attempted by preventing local temperature rise in the filter using a silicon carbide based material having high thermal conductivity and high thermal resistance.
Although the breakage by the thermal stress can be reduced to a certain degree by the segmented structure and/or the use of the materials high in thermal resistance such as a silicon carbide based material, a temperature difference between the outer peripheral portion and central portion of the honeycomb filter cannot be eliminated. Thus a improvement of the regeneration efficiency was insufficient due to insufficient achievement of uniform regeneration, and the heat was locally generated during a regeneration in some cases.
Although, the use of high in thermal conductivity material such as the silicon carbide based material is effective to prevent local temperature rise, the thermal conductivity and porosity of the material are essentially antithetical properties. Therefore, even with the use of the silicon carbide based material, when the porosity is raised in order to reduce a pressure loss as important properties of the filter, the thermal conductivity drops. That is, it has been difficult to simultaneously achieve the reduction of the thermal stress generated by the local heating during the filter regeneration and pressure loss.